In spinning and twisting machines in which sliver or yarn is to be wound upon a take-up spool on being supplied by a tension frame, for example, the transport rollers of the tension frame and the spindle of each take-up spool should maintain a more or less invariable speed ratio in order to maintain the thread material under a predetermined tension. The maintenance of such a speed ratio can be achieved by driving the transport rollers and the spindles by respective synchronous motors which, however, do require a certain adjustment especially in spindle speed throughout the operation in view of the progressively increasing spool diameter. Moreover, it is difficult to maintain the desired speed ratio of such synchronous motors during the startup and cutoff periods.
When squirrel-cage or other 3-phase motors of the asynchronous type are being used to drive these loads, their acceleration times during startup and their deceleration times during cutoff often differ significantly from one another. In particular, a motor with higher power-to-load ratio and with lower operating speed generally attains full speed faster than a motor with lower power-to-load ratio and higher operating speed, provided that both motors are being energized at the same time with the full available supply voltage. Similarly, the return to standstill from their respective operating speeds will take different times from the instant of de-energization, depending on the operating speeds and on the inertia of the associated loads.